Boss monsters are rare enemies that carry a key. They appear instead of a machine room. They are evil analogues to Legendary Allies. The Black Jelly, Goblin Warlord , Vampire and Flame Spirit are all boss monsters.
“The vampire is a boss monster: it appears rarely and guards a key or orb. It is very difficult...Vampire
“The goblin warlord is a boss monster: it appears very rarely and guards rare items such as keys...Goblin Warlord
“The flamedancer is a boss: a rare monster that holds a key or orb. It relies on fire attacks, so...Flamedancer